In recent years, various kinds of devices have been developed in an effort to enhance the driver's convenience and the safe driving. One example of such devices is a head-up display (HUD) for motor vehicles. The head-up display is designed to display a variety of drive information, e.g., a vehicle speed, a revolution per minute and a residual fuel amount, on the windshield of a vehicle.
A flat-panel-type head-up display is primarily used as the head-up display for motor vehicles. Referring to FIG. 1, the flat-panel-type head-up display includes a flat display panel 10 and a backlight unit 12 arranged at the rear side of the display panel 10. The display panel 10 may be one of a liquid crystal display, a liquid-crystal-on-silicon display, a digital micro-mirror display, a thin-film-transistor display and an organic electroluminescence display. The display panel 10 is installed inside of an instrument panel 10a in front of a driver seat so that it can display a drive information image. The backlight unit 12 may be composed of an ultra-high-pressure lamp, a light emitting diode or a laser, and serves as a light source for irradiating light on the rear surface of the display panel 10.
The head-up display further includes an optical system 14 arranged at the front side of the display panel 10. The optical system 14 includes at least one lens 16 positioned at the front side of the display panel 10 and a reflection mirror 18 installed at the front side of the lens 16 at a specified angle. The lens 16 causes the drive information image projected from the display panel 10 to converge or diverge on the reflection mirror 18. The reflection mirror 18 reflects the drive information image coming from the lens 16 toward a windshield 19 so that the drive information image can be displayed on the windshield 19.
Since the head-up display is designed to display a variety of drive information on the windshield 19, it is possible for a driver to readily recognize the vehicle drive information without having to look at the instrument panel 10a. 
In the conventional head-up display, however, the distance L between the display panel 10 and the lens 16 is too short. This poses a problem in that the depth of the drive information image appearing on the windshield 19 becomes too small, which in turn impairs the definition and discrimination of the drive information image observed by a driver. The depth and definition of the drive information image appearing on the windshield 19 can be improved as the distance L between the display panel 10 and the lens 16 increases.
Due to the short distance between the display panel 10 and the lens 16 and the reduced depth of the drive information image, the conventional head-up display suffers from an out-of-focusing phenomenon that the most part of the drive information image other than a particular area looks dim and unclear. This tends to sharply reduce the definition and discrimination of the drive information image.
As a solution to this problem, it would be thinkable to increase the distance L between the display panel 10 and the lens 16. In this case, however, the size of the head-up display becomes too big, consequently making it difficult to install the head-up display within the instrument panel 10a. 